1. Field of the Invention
This invention relates to NMR imaging. In a primary application it relates to defining the NMR parameters of all regions of a volume through simultaneous data acquisition.
2. Description of Prior Art
In recent years significant attention has been focused onto the problem of NMR imaging. Here the fundamental problem is that of spatial localization where various NMR parameters are measured at specific local regions within a volume. One of the most significant contributions to the localization problem was the sensitive-point method of Waldo S. Hinshaw. This is described in a paper by Hinshaw entitled, "Image Formation by Nuclear Magnetic Resonance: The Sensitive-Point Method," in the Journal of Applied Physics, Vol. 47, pp. 3709-3721, August 1967 and in British Pat. No. 1,508438 and U.S. Pat. No. 4,015,196 issued to W. S. Moore and W. S. Hinshaw.
The fundamental method involves the use of time-varying or a.c. gradient fields. In its simplest embodiment a.c. gradient fields of different frequencies are applied to all three axes. The demodulated signal is then integrated so that all temporal variations are removed. The resultant integrated signal is therefore sensitive only to the null region of the various gradient fields. One point in space, corresponding to the null of all three a.c. gradient fields, provides an integrated output signal. To provide an image, the a.c. gradient fields are altered to move the null point. This method is effective, but slow, since one point at a time is acquired, and each point requires significant integration time.
It is interesting to note that, in British Pat. No. 1,508,438 the inventors indicate that each point in the volume experiences a unique time dependency which is distinguishable from signals produced by other points in the volume. However, in all published material to this date, no method has been shown for studying any point other than the null point; thus making the method effective, but very slow. Its unusually slow speed has kept it from use in generalized image applications where other methods have been dominant. Its only present day use has been that of localized spectrometry where the NMR spectrum of any desired local region can be studied. One example of this is in a paper by Katherine N. Scott, et al. entitled, "Spatial Localization of .sup.31 P Nuclear Magnetic Resonance Signal by the Sensitive Point Method," appearing in the Journal of Magnetic Resonance, Vol. 50, pp. 339-344, 1982.
As previously indicated, the use of three a.c. gradient signals followed by integration isolates a specific point. Similarly, two a.c. gradients isolate a line and one a.c. gradient isolates a plane, using the same null phenomenon. The use of lines or planes can be part of various combined imaging systems such as those involving reconstruction from projections. In addition, it should be pointed out that two of the a.c. gradient fields can be of the same frequency but shifted by 90.degree. in phase, providing the required orthogonality. An interesting variation on the sensitive point method is described in British Pat. No. 1,601,816 invented by Waldo S. Hinshaw where a line array of points are acquired simultaneously. Here a.c. gradients, such as two orthogonal sinusoids, are applied to two axes with a static gradient on the third axis. The filtering of the a.c. signals limits the acquisition to the line defined by the intersection of the null planes. However, due to the static gradient, each point along the null line represents a different frequency. Thus Fourier transformation of the filtered signal provides simultaneous information about points along the null line. However, no method is shown of studying the activity of points in other lines other than changing the a.c. gradients and decomposing a new line, with its attendant problems of long acquisition time.
One method, however, does provide for the simultaneous acquisition of data of points within a plane. This method, known as the echo planar system is described in a paper by P. Mansfield and I. L. Pykett in the Journal of Magnetic Resonance, vol. 29, p. 355, 1978. It is also described in the book by P. Mansfield and P. G. Morris NMR Imaging in Biomedicine, Academic Press, 1982. In this method an xy plane is excited and, while the resultant signals are recieved, a static gradient is applied in the x dimension and a square wave gradient in the y dimension. The square wave gradient essentially involves amplitude modulation of each region at a frequency based on its y position. Because of the periodic modulation, discrete regions along y are received each representing a different frequency. These discrete y positions are superimposed on a continuous frequency spectrum representing the x coordinates due to the static gradient. Thus each frequency represents a spatial position with all of the spatial information acquired simultaneously.
This system has a number of problems. Firstly, the modulation technique, resulting in discrete frequencies, limits the data acquisition to discrete positions in the y dimension rather than the desired ability to access all regions. This modulation technique also limits the matrix size or system resolution as pointed out by Mansfield. Also, although in theory the method is applicable to acquiring all three dimensions simultaneously by using an additional modulated z gradient, this would result in severe spectral complexity and has yet to be attempted as far as published literature is concerned.
The limitation of the technique essentially lies in the fact that each spatial position is represented by a specific region of the frequency spectrum of the signal.
A variation on the echo planar system was described in a paper by M. M. Tropper in the Journal of Magnetic Resonance, vol. 42, pp. 193-202, 1981 entitled "Image Reconstruction for the NMR Echo-Planar Technique, and for a Proposed Adaptation to Allow Continuous Data Acquisition." As with the echo planar system, data is acquired from a single plane using one static and one time-varying gradient during the receiving time. The signal processing, however, makes more efficient use of the signal. The specific processing system shown, however, is quite complex in that it involves a unique sampling sequence followed by a Fourier transform for each image point. It does, however, provide improved performance over the original echo planar method. The paper does not discuss simultaneous acquisition of information from the entire volume.